In the fields of wideband signal transmission and data communications, it is possible that reception of the transmitted signal will be disrupted due to the presence of an unwanted tone or other narrowband interference source. For example, a slowly changing amplitude or frequency modulated carrier is a possible source of narrowband interference.
Various systems have been devised to eliminate, or to diminish, the effect of these interference sources. In particular, with the use of what are called single tone cancellers and adaptive line enhancers, the reduction of certain types of interference has been effectuated.
In cases where a desired wideband signal is corrupted by the presence of one or more interfering tones, such as may often occur in a communication system, an adaptive line enhancer (ALE) could be utilized. These devices are effectively adaptive noise cancellers which actually create their own reference signal, rather than rely on a supplied reference signal. An ALE can handle multiple tone interference. A single tone canceller can handle one tone.
As will be shown, a single tone canceller requires a noise-free "reference" tone which is phase related to the single tone interference frequency which is disturbing the transmission.
The reference tone required by single tone adaptive noise cancellers may be generated with a synthesizer circuit as long as the frequency of the interference source is precisely known ahead of time.
However, in many situations where single tone interference is a problem, the frequency is not known beforehand, or additionally, the frequency of the interference signal may be varying slowly with time. Under these conditions, the use of a synthesizer becomes an impractical application.
In the adaptive line enhancer, the creation of its own reference signal is accomplished by "delaying" a copy of the input signal to "decorrelate" the wideband component and then to filter this delayed signal with a transversal filter. This transversal filter attenuates the wideband component while adjusting the phase and amplitude of the narrowband component to match that of the narrowband interference (NBI) which is contained in the input signal.
This matched narrowband signal is then subtracted from the entire wideband input signal, thus cancelling the narrowband interference.
It might be noted, however, that transversal digital filters are extremely expensive, and in the case where there is only a single tone interferer, they would be considered unnecessary.
A much more effective and efficient embodiment is provided herein which involves the use of a phase-locked loop (PLL) to generate the required reference tone.
The situation occurs where wideband signals of interest are corrupted by the presence of single tone interference signals. A solution is provided by a system in which a circuit regenerates the single tone interference which then is used to exactly cancel the original single tone interference from the wideband input signal, and thus leave the desired interference-free wideband signal.
In the past, when a clean reference of the single tone interference was available, adaptive single tone cancellers have been used. These single tone cancellers were limited in their usefulness according to the availability of a "clean", or noise-free, reference signal.
In the present disclosure, a phase-locked loop (or a frequency-locked loop) is used to generate this "clean" reference signal. Thus, the previous limitation of requiring a clean reference signal is no longer necessary since the phase/frequency-locked loop provides the clean reference for the single tone adaptive noise canceller. As stated, the single tone canceller requires a clean reference tone. This clean reference tone is provided by the phase/frequency-lock loop. But the phase and amplitude of this reference tone must be adjusted before cancellation of the interference can be achieved.
This adjustment is accomplished by the two-weight adaptive noise canceller, just as is done when a reference tone is provided. The procedure involved in adjusting the phase and amplitude is "adaptive" and therefore requires feedback.
One very widely used adaptation algorithm is the LMS (least mean square) algorithm. Certain implementations of this algorithm introduce time delays into the feedback loop, and these time delays must be compensated for.
In the prior art, as seen in FIG. 3, analog (or digital) transversal adaptive filters were used, whereby the wideband signal plus the interference signal I.sub.s was fed into the point A. This led to an analog (or digital) decorrelation delay and thence to a tapped delay line which handled, typically, 16 tap points. The adaptive line enhancer shown in FIG. 3 functions specifically by using the LMS algorithm.
Every single one of the 16 taps has an output line which was fed into a multiplier; e.g., M.sub.P and thence to an integrator I.sub.P. The output of integrator I.sub.P is referred to as a "filter weight", and multiplies the same tap value. Each tap has a corresponding weight which "weights" the tap value at multiplier M.sub.2. In each case for each of the 16 taps, the output of M.sub.2 was fed to a summer S.sub.P where the regenerated narrowband interferer appears. The output of this summer was fed on line B to a subtracter circuit ST.sub.P which also received the initial input from line A. After convergence, the narrowband interference is nulled out at the subtracter circuit ST.sub.P, leaving the wideband signal on output line L.sub.o clean and clear of the interference signal I.sub.S.
The multiplier M.sub.P additionally received a feedback of the output from L.sub.o along line C. The process of multiplying this feedback signal by the tap signal at multiplier M.sub.P, and then integrating with integrator I.sub.P, provides the filter weights. (LMS algorithm)
However, this type of system involved multitudes of components,--for example, two multipliers and one integrator per tap. Thus, for 16 taps, this came to 3.times.16=48 units plus 16 input summer circuits, plus 16 tapped delay line elements plus a decorrelation delay unit. The present embodiment shown in FIG. 1 reduces, in great measure, the number of components involved to four multipliers, two integrators, two summer circuits, a phase shifter and a phase/frequency-lock loop.
A need for extending the utility of the single tone canceller to very high frequency bands arose.
Digital implementations of the two-weight adaptive noise canceller are desirable because of the precision and predictability provided by currently available digital circuitry. However, the operating band of digital filters is restricted to baseband, in particular from DC to one-half the clock frequency of the digital circuitry (Nyquist's theorem).
If it is desired to filter a frequency band which lies outside this baseband range, then an analog/digital implementation (FIGS. 4, 5, 6) may be considered. Here, the two-weight adaptive filter (of FIG. 2) is digital but the phase/frequency-lock loop is analog.